calcination gives higher crystallinity, smaller specific surface
area and rutile crystallites, while lower-temperature calcination
gives lower crystallinity, larger specific surface area and anatase
crystallites. It has been reported that photocatalytic activity of
titania particles prepared in such a way decreased drastically
at the temperature at which anatase–rutile transformation
occurred. Since both crystalline form and specific surface area
were changed drastically at the same time and there have been
no reported ways to extract the intrinsic effect of each property,
it is difficult to determine which property (or both of them) is sig-
nificant. Discussions on property–activity correlations reported
so far, including those reported by a group of the author, may
involve such a problem. It can be said that rutile titania samples
with a small surface area that are prepared at a high tempera-
ture show low photocatalytic activity, but it is scientifically (logi-
cally) impossible to state that the conversion of crystalline form
or drastic reduction of specific surface area is the reason for the
low photocatalytic activity.
A.3. Extraction of intrinsic effects of physical and structural properties
A plausible method to extract the intrinsic effect of each physi-
cal and structural property is statistical analysis of data on phys-
ical and structural properties and photocatalytic activities for
samples of the same composition, such as titania. In a recent
study by the author’s research group, photocatalytic activities
and physical and structural properties of 35 commercial titania
powders were statistically analyzed to find the predominant
property (properties) determining the activity of a given reaction
system ( 50 ): standardized photocatalytic activities for five kinds
of reactions were fairly well reproduced by a linear combination
of six kinds of physical and structural properties of
photocatalysts, that is, specific surface area, density of crystal-
line defects, primary particle size, secondary particle size, and
existence of anatase and rutile phases. Recently, decahedral-
shaped anatase titania particles (DAPs) have been prepared by
controlled gas-phase reaction of titanium(IV) chloride and oxy-
gen at 1473 K (Fig. 13) ( 51 ). The photocatalytic activity of DAPs
was reported to be much higher than the photocatalytic activities
of commercial titania particles, for example, Degussa (Evonic)
P25, presumably due to relatively large specific surface area to
adsorb a large amount of the substrate(s) and high crystallinity,
that is, less crystalline defects to reduce e––hþrecombination.
Then, how does the decahedral shape itself affect the
photocatalytic activity? It was suggested that high levels of
422 B. OHTANI